The invention relates to the field of communication systems, and in particular to direct sequence spread spectrum systems utilizing differential orthogonal modulation.
Modulation of radio signals using direct sequence spread-spectrum codes is a widely used technique in communications systems. The advantage of the technique is that multiple signals can share the same frequency space, or channel, without interference.
In a typical DSSS communications system, data units (typically binary) are represented by codes, where each code is itself a sequence of 1 s and 0s. The code sequences are selected from a special class of sequences known as pseudo-noise (PN) sequences, which have the properties of low self correlation and low cross correlation with other codes. At the receiver, the binary data is recovered by correlating the received data with a set of desired codes. Signals applied to the receiver that are not encoded with the desired PN codes are not correlated. The action of the correlator allows a desired PN-coded signal to be received in the presence of stronger uncorrelated signals. The gain associated with the correlation process (spreading gain) is proportional to the length of the code (in chips, or binary elements).
The most common application of DSSS communications systems is to facilitate multiple access, or multiple transmissions sharing the same frequency space, location space and time. However, if multiple access is not the goal, it is possible to use DSSS as a means of increasing the data rate in a given channel compared to traditional means. It is this concept that is the focus of the present invention.
Use of orthogonal codes as a modulating means is a concept that has been discussed in U.S. Pat. No. 6,212,219 B1 and in U.S. patent application Ser. No. 09/803,258. In the concept as presented in these references, data is coded in a multi-bit format using multiple PN codes. The concept is extended through the use of time shifts on the codes. Codes are selected such that they are orthogonal (zero cross correlation for all times), thus enabling the time shifted codes to be used as modulating elements.
As an example of the power of this method, consider the case of a binary data stream modulating a carrier with one of 4 orthogonal codes, each with 16 possible time shifts. If a symbol is defined as a single instance of an orthogonal code sequence, it can be seen that 6 bits (4 codes, each with 16 possible time shifts) can be represented by each symbol. This is a 6xc3x97 improvement in throughput over traditional DSSS modulation. Furthermore, because the codes are orthogonal, the increase in signal power required to overcome channel noise is less than 2 dB higher than is required for the single bit case. This is a large improvement over traditional DSSS modulation, where a 6xc3x97 increase in bit rate would result in a 10*log(6)=7.5 dB increase in required transmit power to achieve a given signal to noise ratio.
As the above example demonstrates, orthogonal modulation methods are powerful for achieving high energy efficiency in transmission. One of the motivations of this invention is to capture this advantage.
A key limitation in existing schemes is in the way that time-shifted versions of codes are recognized. Related art proposes the use of a quadrature modulation scheme, where the I-channel is used as a time reference and a Q-channel is used to transmit data. Time shifted code sequences are recognized by comparing the start of the reference code sequence on the I-channel with the start of a data code sequence on the Q-channel. This approach, while workable, has two disadvantages:
1) Two channels are utilized but only one channel carries data. The result is that maximum bit rate of the channel is reduced by a factor of 2.
2) Timing of sequence shifts on the Q-channel depends on identical propagation times of the I and Q channels. If this is not the case, errors result.
Accordingly, there is need for a communications system that maximizes the bit rate for a given bandwidth and sensitivity by allowing single-channel transmission, allowing quadrature systems with data on both I and Q channels (2xc3x97 capacity improvement), and eliminating the need for timing between I and Q channels.